Modular glenoid assembly

ABSTRACT

A modular glenoid assembly is provided for attachment to a glenoid surface of a scapula. The modular glenoid assembly includes a base adapted to couple with the glenoid surface and a bearing insert. The base includes a lip that defines a channel. The bearing insert includes a bearing surface adapted to engage a head portion of a humeral component and a tab. The tab is formed to be received within the channel and engage the lip when the insert is moved in a superior direction into position against the base.

This application claims priority under 35 U.S.C. §119(e) of Ser. No.60/088,594 filed Jun. 9, 1998 in the United States Patent and TrademarkOffice and Ser. No. 60/094,757 filed Jul. 31, 1998 in the United StatesPatent and Trademark Office.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a prosthetic glenoid component andparticularly to a modular glenoid assembly for attachment to a glenoidsurface of a scapula to replace a natural socket of a shoulder and toprovide a bearing surface for a head portion of an arm bone or humerus.More particularly, the present invention relates to a modular glenoidassembly comprising a metal backed base coupled to a plastic bearinginsert.

According to the present invention a modular glenoid assembly isprovided that comprises a base adapted to couple with the glenoidsurface and a bearing insert providing a bearing surface adapted toengage the head portion of the humerus. The base is provided with afirst connector portion and the insert is provided with a secondconnector portion. In addition, the first and second connector portionsare engageable and proportional to secure the insert to the base whenthe insert is moved into engagement with the base.

In preferred embodiments, the first connector portion includes a channelextending about a portion of the base and the second connector portionincludes a tab extending about a portion of the insert. The tab isreceived in engagement with the channel when the insert is moved in asuperior direction into position against the base. In addition, thechannel includes inferiorly and inwardly opening portions and the tabincludes superiorly and outwardly extending portions to be receivedrespectively in the inferiorly and inwardly extending portions.

Further, according to the present invention a modular glenoid assemblyfor attachment to a glenoid surface of a scapula to provide a bearingsurface is provided that comprises a base adapted to couple with theglenoid surface and a bearing insert. The base includes a lip and thebearing insert includes a bearing surface adapted to engage the headportion of the humeral component and a tab formed to engage the lip whenthe insert is moved in a superior direction into position against thebase.

Still further in accordance with the present invention a modular glenoidassembly is provided that comprises a metal base and a plastic bearinginsert. The metal base includes a lower surface, an upper surface, atleast one peg extending from the lower surface, and a lip extendingabout a portion of the upper surface and defining a channel. The bearinginsert includes bearing surface, an opposite lower surface facing theupper surface of the base, and a tab extending about a portion of thelower surface. The tab is received in the channel from an inferior to asuperior direction to mount the insert on the base.

Additional features of the present invention will become apparent tothose skilled in the art upon consideration of the following detaileddescription of preferred embodiments exemplifying the best mode ofcarrying out the invention as presently perceived.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a modular glenoid assembly inaccordance with the present invention positioned between a glenoidsurface of a scapula and a humeral component showing the glenoidassembly including a base having upper and lower surfaces, pegsextending from the lower surface, a recessed portion formed in the uppersurface, and a lip extending about a portion of the upper surface abearing insert formed to include a bearing surface and a lower surfaceincluding a tab engagable and proportional to the lip;

FIG. 2 is an assembled sectional view taken along lines 2—2 of FIG. 1showing the pegs of the base received within apertures formed in theglenoid surface and the bearing insert mounted on the base positioningthe bearing surface in engagement with the humeral component;

FIG. 3 is a sectional view taken along lines 3—3 of FIG. 2 showing thelip of the base including a channel and the tab of the insert receivedin the channel;

FIG. 4 is a bottom view of the bearing insert of FIG. 3 showing thelower surface of the insert including an inferior end, a superior end,and a central portion, the detent extending from the central portion andthe tab extending about the superior end and central portion of theinsert;

FIG. 5 is a view similar to FIG. 1 of an alternative embodiment of abase that is suitable for use with the insert showing the base havingupper and lower surfaces, two apertures extending between the upper andlower surfaces for receiving screws in order to couple the base to thescapula, a peg extending from the lower surface of the base, a lipextending about a portion of the upper surface, and a recessed portionformed in the upper surface for receiving the detent of the insert;

FIG. 6 is a view similar to FIG. 2 of the base and insert of FIG. 5showing the peg and the screws received in the scapula and the lip ofthe base including a channel that receives the tab to couple the bearinginsert to the base;

FIG. 7 is a sectional view of an alternative embodiment of a glenoidassembly in accordance with the present invention showing a baseincluding a generally flat upper surface, a lower surface, a pegextending from the lower surface, and an outer lip extending about aportion of the upper surface and a bearing insert including a generallyflat lower surface configured for mating with the upper surface of thebase;

FIG. 8 is a sectional view of the base of FIG. 7 showing screw holesextending through the base between the upper and lower surfaces and thepeg positioned to lie between the spaced-apart screw holes;

FIG. 9 is a bottom view of the bearing insert of FIG. 7 showing thelower surface of the insert including an inferior end, a superior end,and a central portion and the tab extending about the superior andcentral portion of the insert; and

FIG. 10 is a view taken along lines 10—10 of FIG. 9 showing the bearinginsert having a generally flat lower surface.

DETAILED DESCRIPTION OF THE DRAWINGS

A modular glenoid assembly 10 is provided in accordance with the presentinvention. FIG. 1 shows glenoid assembly 10 located between a glenoidsurface 16 of a scapula 18 and a head portion 22 of a humeral component24. The glenoid surface 16 has an inferior portion 16 i and a superiorportion 16 s as shown in FIG. 1. Glenoid assembly 10 is designed to beattached to glenoid surface 16 of scapula 18 to replace the naturalglenoid surface. Glenoid assembly 10 includes a base 12 provided with afirst connector portion and a bearing insert 14 provided with a secondconnector portion that is slidably received by first connector portionof base 12 along an axis 26 in a superior direction 28. First and secondconnector portions are engageable and proportional to secure the insert14 to base 12 when insert 14 is moved into engagement with base 12.Thus, glenoid assembly 10 resists physiologic loading by rotator cuffand deltoid muscles (not shown).

Base 12 of modular glenoid assembly 10 is formed to include a body 29that has a concave upper surface 30 positioned to face insert 14 and aconvex lower surface 32 positioned to face glenoid surface 16. See FIG.1. Superior, inferior, and center attachment pegs 40, 42, 44,respectively, are formed to extend from lower surface 32 to anchor body29 to glenoid surface 16. A right stabilizing peg 46 and a similarlyformed left stabilizing peg (not shown) additionally extend from lowersurface 32.

Attachment pegs 40, 42, 44 each include a generally cylindrical bodyportion 48 and a generally conical tip portion 50, as shown in FIG. 1.Stabilizing pegs 46 also include a generally cylindrical body portion 52and a generally conical tip portion 54. Glenoid surface 16 is formed toinclude apertures 60 aligned for receiving respective attachment pegs40, 42, 44 and stabilizing pegs 46. Tip portions 50, 54 facilitateinsertion of the pegs 40, 42, 44, 46 into apertures 60 of glenoidsurface 16. The diameter of body portions 48, 52 is slightly less thanthe diameter of apertures 60 in order to provide space for a cementmantle (not shown) around the inner circumference of apertures 60 or toallow for interference fit. It is understood that pegs 40, 42, 44, 46may be a wide variety of shapes and sizes in accordance with the presentinvention.

As shown in FIG. 2, superior and inferior pegs 40, 42 are each formed toinclude a plurality of notches or serrations 62. Serrations 62 increasethe area of contact between pegs 40, 42 and the cement mantle (notshown) placed within apertures 60 to strengthen the bond between pegs40, 42 and the bone of glenoid surface 16. Referring again to FIG. 1,each stabilizing peg 46 is also formed to include notches or serrations62 so that the bone cement may secure stabilizing pegs 46 to the bone ofthe glenoid surface 16. It is within the scope of the present inventionto form serrations 62 on one or more pegs 40, 42, 44, 46. It is alsowithin the scope of the present invention to include pegs 40, 42, 44, 46having a porous coating also for strengthening the bond between the pegsand the bone of glenoid surface 16. A more detailed description of pegs40, 42, 44, 46 may be found in U.S. Pat. No. 5,032,132, entitled“Glenoid Component”, to Frederick A. Matsen, III, which is incorporatedherein by reference. While pegs 40, 42, 44, 46 are illustrated anddescribed, it is within the scope of this disclosure to use a widevariety of attachment mechanisms to couple base 12 and scapula 18together.

Referring now to FIG. 1, upper surface 30 of body 29 is formed forsliding engagement with bearing insert 14. Upper surface 30 includes asuperior end 64, an inferior end 66, a center portion 68 positioned tolie between superior and inferior ends 64, 66, and a perimeteral edge72. In addition, a first connector portion or outer lip 70 extends fromperimeteral edge 72 adjacent to superior end 64 and center portion 68.As shown in FIGS. 2 and 3, outer lip 70 includes a side wall 73extending from upper surface 30 and a tab 75 extending from side wall 73over upper surface 30 to define a channel 74. Tab 75 includes a taperedsurface 77 that faces insert 14 (See FIG. 3). Channel 74 is defined byupper surface 30 and lip 70 and includes inferiorly and inwardly openingportions. Channel 74 is generally U-shaped and extends, as stated above,only partially about perimeter edge 72 so that inferior end 66 of base12 is spaced apart from lip 70.

As shown in FIG. 2, center portion 68 of upper surface 30 is also formedto include a recessed portion 80. Recessed portion 80 cooperates withbearing insert 14 to provide the surgeon with the feel of snap-fit.Recessed portion 80 is generally oval in shape, although it is withinthe scope of this disclosure to form recessed portion in a wide varietyof shapes and patterns within upper surface 30. It is within the scopeof this disclosure to form upper surface 30 without recessed portion,with greater than one recessed portion, or with a detent (not shown)thereon to cooperate with a recessed portion in insert. Base 12 isconstructed from metal such as titanium alloy, however, it is within thescope of this disclosure to form base from a wide variety of metals suchas cobalt chrome alloy and stainless steel. In addition, base 12 ispreferably highly polished.

Bearing insert 14 is formed for sliding engagement with base 12 andprovides a bearing surface 20 for head portion 22. As shown in FIG. 1,insert 14 includes a concave upper surface 82 facing head portion 22 andan opposite convex lower surface 84. Upper surface 82 of insert 14provides bearing surface 20 for head portion 22 of humeral component 24as shown in FIGS. 2, 4, and 5. Upper surface 82 includes a superior end102, an inferior end 104, and a central portion 106 positioned to liebetween superior and inferior ends 102, 104. An insert-perimeter edge108 extends between upper and lower surfaces 82, 84. Edge 108 is formedto include a groove 90 therein. Groove 90 extends into perimeter edge108 adjacent to superior end 102 and central portion 106 and is sized toreceive tab 75 of lip 70 therein. As shown in FIG. 3, groove 90 isdefined by a bottom surface 101, a top surface 111 and a side wall 113.Groove 90 is generally U-shaped and extends, as stated above, onlypartially about edge 108 so that inferior end 104 is spaced apart fromgroove 90. When assembled, groove 90 and channel 74 are situated in ageneral stacked alignment with one another.

As shown in FIG. 3, when assembled, lower surface 84 of insert 14 restsupon upper surface 30 of base 12. Lower surface 84 includes a superiorend 105, an inferior end 103, and a central portion 109 positioned tolie between superior and inferior ends 105, 103. See FIG. 4. Lowersurface 84 is also formed to include a second connector portion or tab88 that extends into channel 74 and engages lip 70 and upper surface 30.Tab 88 extends adjacent to groove 90 about superior end 105 and centralportion 109 of lower surface 84. Tab 88 has a top end 94 and a bottomend 95 at inferior end 103 of lower surface 84. Thus, tab 88 includessuperiorly and outwardly extending portions that are receivedrespectively in the inferiorly and inwardly extending portions ofchannel 74.

Once base 12 and insert 14 are assembled, lower surface 84 of insert 14rests upon upper surface 30 of base 12. Insert 14 also includes a detent92 extending from central portion 109 of lower surface 84. Detent 92cooperates with recessed portion 80 to provide the feel of a snap-fitassembly. Detent 92 is rectangular in shape, although it is within thescope of this disclosure to form detent 92 in any number of shapes orsizes having greater or fewer than one detent, or form lower surface 84to include a recess to interact with a corresponding detent (not shown)on base 12. Insert 14 is constructed of ultra high molecular weightpolyethylene (UHMWPE), however, it is within the scope of the inventionto manufacture insert 14 from a wide variety of materials suitable fororthopaedic use.

In assembling glenoid assembly 10, insert 14 is moved upward or insuperior direction 28 along axis 26, as shown in FIG. 1. Tab 88 firstengages inferior end 66 of upper surface 30. As insert 14 slides indirection 28 relative to base 12 along axis 26, tab 88 is receivedwithin channel 74 of base 12, as shown in FIGS. 2 and 3. Additionally,tab 88 of lower surface 84 is received within channel 74 and tab 75 ofouter lip 70 of base 12 is received within groove 90 of insert 14.

Tab 88 of lower surface 84 moves in direction 28 within channel 74 untiltop end 94 of tab 88 engages side wall 73 of lip 70 adjacent to superiorend 64. Detent 92 is formed to be received by recessed portion 80 ofbase 12 so that during assembly, detent 92 “snaps” into engagement withrecessed portion 80 notifying the surgeon that insert 14 is mounted onbase 12. Side wall 73 of outer hip 70 at superior end 64 also acts as astop for top end 94 of tab 88. Once insert 14 is mounted on base 12, asshown in FIGS. 2 and 3, bearing surface 20 is ready to engage headportion 22 of humeral component 24. It is also within the scope of thepresent disclosure to include a ramp (not shown) on upper surface 30 fordetent 92 to slide upon in order to more easily guide detent 92 intorecessed portion 80.

Once implanted within glenoid surface 16 of scapula 18, glenoid assembly10 encounters loading forces from rotator cuff muscles (not shown) andthe deltoid muscle (not shown). As shown in FIG. 2, these loading forcesare generally oriented in a superior direction 97. Glenoid assembly 10may also encounter forces in an anterior/superior direction 98 or aposterior/superior direction 99, as shown in FIG. 1. Because of thegenerally superior direction 97 of the loading on glenoid assembly 10,insert 14 is prevented from disengaging base 12.

An alternative embodiment of glenoid assembly 110 in accordance with thepresent invention is shown in FIGS. 5 and 6. Assembly 110 includes abase 112 that slidingly receives bearing insert 14 along an axis 121 ina superior direction 123. Base 112 is formed similarly to base 12 andlike reference numerals will be used to denote like components. Base 112includes an attachment peg 144 that extends from lower surface 32adjacent to center portion 68. Peg 144 is formed in a manner similar topegs 40, 42, 44. It is appreciated, however, that peg 144 may be formedin a variety of shapes and sizes in accordance with the presentinvention. Peg 144 is received by pre-formed aperture 60 in glenoidsurface 16. As shown in FIG. 5, body 29 includes two holes 120therethrough that are sized to receive corresponding screws 122. Asshown in FIG. 6, when fully assembled, screws 122 extend through holes120 and into pre-formed apertures 160 in glenoid surface 16. Thus,screws 122 cooperate with peg 144 to couple base 112 to glenoid surface16 of scapula 18. Base 112 is constructed of materials similar to base12.

The assembly method of glenoid assembly 110 is similar to that describedabove in reference to glenoid assembly 10. Insert 14 slides alongsuperior/inferior axis 121 in direction 123 so that tab 88 of lowersurface 84 is received within channel 74 of base 112. At this time, tab75 of lip 70 is received within groove 90 of insert 14. It becomesevident to the surgeon that insert 14 received within base 112 whendetent 92 and recessed portion 80 produce a snap-in feel.

An additional embodiment of the present invention is shown in FIGS.7-10. As shown in FIG. 7, glenoid assembly 210 is provided that includesa base 212 and a bearing insert 214. Base 212 of glenoid assembly 210 iscoupled to glenoid surface 16 in a manner similar to base 112. Base 212includes a body 229 that has a lower surface 232 configured forengagement with glenoid surface 16 and a generally planar upper surface230. Upper surface 230 includes a superior end 264, an inferior end 266,and a center portion 268 positioned to lie between superior and inferiorends 264, 266. An outer lip 270 extends from upper surface 230 aboutsuperior end 264 and center portion 268. Lip 270 is formed in a mannersimilar to lip 70 of base 12, as previously described and includes achannel 274 therein. Channel 274 is generally U-shaped and formed in amanner similar to channel 74. In addition, as shown in FIG. 7, uppersurface 230 of base 212 is formed to include a recessed portion 280therein. Recessed portion 280 is similar to recessed portion 80 andcooperates with insert 214 to provide the snap-in feel previouslydescribed.

Referring now to FIG. 8, body 229 includes spaced apart screw holes 320therethrough. Screw holes 320 extend between lower and upper surfaces232, 230 and are spaced apart about an attachment peg 244. Screw holes320 are sized for extension of screws (not shown) therethrough. It iswithin the scope of the present disclosure that base 212 be formed withgreater or fewer than two screw holes 320. Peg 244 extends from lowersurface 232 of base 212 and is formed to be received within pre-formedapertures 60 (see FIG. 5) within glenoid surface 16. Screws (not shown)extend through holes 320 of base 212 and into apertures 160 (see FIG. 5)formed in glenoid surface 16.

Turning now to FIGS. 9 and 10, bearing insert 214 cooperates with head22 in a manner similar to insert 14 as previously described. Insert 214includes a generally planar lower surface 284 and an opposite concaveupper surface 282. Upper surface 282 is formed in a manner similar toupper surface 82 as previously described. Lower surface 284 includes asuperior end 203, an inferior end 205, and a center portion 209positioned to lie between superior and inferior ends 203, 205. A tab 288is formed in lower surface 284. As shown in FIG. 9, tab 288 includes atop end 294 and a bottom end 295.

The assembly method of glenoid assembly 210 is similar to that describedabove in reference to glenoid assemblies 10, 110. Insert 214 slidesalong superior/inferior axis 221 in direction 223 so that tab 288 isreceived within channel 274 of base 112. At this time, lip 270 isreceived within groove 290 of insert 214. It becomes evident to thesurgeon that insert 214 is received within base 212 when detent 292 andrecessed portion 280 of base 212 produce a snap-in feel.

To couple assemblies 10, 110, 210 to scapula 18 the surgeon performs thefollowing steps. Scapula 18 is sized and reamed in accordance with wellknown surgical procedures. At least one hole 60 is drilled withinglenoid surface 16 to correspond with peg(s) on base 12, 112, 212respectively. Base 12, 112, 214 is implanted within scapula 18 withchannel 70, 270 opened inferiorly. Insert 14, 14, 214 is then slid intorespective base 12, 112, 212 from inferior to superior direction. Thus,locking mechanism of the present invention utilizes a tongue-in-grooveor tab-in-slot design. The metal base 12, 112, 214 is oriented such thatthe insert 14, 14, 214 slides in from an inferior to superior directiononce the metal base 12, 112, 212 is securely fixed to glenoid 18.

Although the invention has been described in detail with reference to apreferred embodiment, variations and modifications exist within thescope and spirit of the invention as described and defined in thefollowing claims.

What is claimed is:
 1. A modular glenoid assembly for attachment to aglenoid surface of a scapula to provide a bearing surface, the modularglenoid assembly comprising: a base having (i) a lower surface adaptedto couple with the glenoid surface, (ii) an upper surface, (iii) asuperior end, (iv) an inferior end, (v) a center portion, (vi) aperimeteral edge, and (vii) a lip which defines a channel, wherein thelip extends around a portion of the perimeteral edge at the superiorend; and a bearing insert including (i) a bearing surface adapted toslidably engage a head portion of a humeral component, and (ii) a tabformed to be engaged within the channel of the lip when the insert ismoved into position against the base, wherein both the lip and thechannel are generally U-shaped.
 2. A modular glenoid assemblycomprising: a metal base including a lower surface, an upper surface, aninferior end, a superior end, a center portion, a perimeteral edge, atleast one peg extending from the lower surface, and a lip extendingabout a portion of the perimeteral edge at the superior end and defininga channel; and a plastic bearing insert including a bearing surfaceadapted to slidably engage a head portion of a humeral component, anopposite lower surface facing the upper surface of the base, and a tabextending about a portion of the lower surface, the tab being receivedin the channel from a first direction to a second direction to mount theinsert on the base, wherein both the lip and the channel are generallyU-shaped.
 3. A method of coupling a glenoid assembly to a glenoidsurface of a scapula, the glenoid surface having a superior portion andan inferior portion, comprising the steps of: providing a base of theglenold assembly, the base including a first connector portion; couplingthe base to the glenoid surface; providing an insert of the glenoidassembly, the insert being configured to contact a head portion of ahumeral component, and further the insert having a second connectorportion; and engaging the first connector portion with the secondconnector portion so as to couple the insert to the base, wherein (i)movement from the inferior portion to the superior portion of theglenoid surface defines a superior direction, and (ii) the insert ismoved from the inferior portion toward the superior portion in relationto the base in the superior direction during the engaging step.
 4. Themethod of claim 3, further comprising the step of providing loadingforces to the glenoid assembly in the superior direction with a muscleafter the insert is coupled to the base.
 5. The method of claim 4,wherein the muscle is a deltoid muscle or a rotator cuff muscle.
 6. Themethod of claim 3, wherein: the first connector portion includes achannel extending about a portion of the base, the second connectorportion includes a tab extending about a portion of the insert, and theengaging step includes the step of receiving the tab within the channelwhen the insert is moved in relation to the base in the superiordirection.
 7. The method of claim 3, wherein: the base includes agenerally concave upper surface, the insert includes a generally convexlower surface, and the generally concave upper surface contacts thegenerally convex lower surface during the engaging step.
 8. The methodof claim 3, wherein: the base includes a generally flat upper surface,the insert includes a generally flat lower surface, and the generallyflat upper surface contacts the generally flat lower surface during theengaging step.
 9. A method of coupling a glenoid assembly to a glenoidsurface of a scapula, the glenoid surface having a superior portion andan inferior portion, comprising the steps of: providing a base of theglenoid assembly, the base including a lip; coupling the base to theglenoid surface; providing an insert of the glenoid assembly, the insertbeing configured to contact a head portion of a humeral component, andfurther the insert having a tab; and engaging the tab within the lip soas to couple the insert to the base, wherein (i) movement from theinferior portion to the superior portion of the glenoid surface definesa superior direction, and (ii) the insert is moved from the inferiorportion toward the superior portion in relation to the base in thesuperior direction during the engaging step.
 10. The method of claim 9,wherein: the lip defines a channel which extends about a portion of thebase, and the engaging step includes the step of receiving the tabwithin the channel when the insert is moved in relation to the base inthe superior direction.
 11. The method of claim 9, further comprisingthe step of providing loading forces to the glenoid assembly in thesuperior direction with a muscle after the insert is coupled to thebase.
 12. The method of claim 11, wherein the muscle is a deltoid muscleor a rotator cuff muscle.
 13. The method of claim 9, wherein: the baseincludes a generally concave upper surface, the insert includes agenerally convex lower surface, and the generally concave upper surfacecontacts the generally convex lower surface during the engaging step.14. The method of claim 9, wherein: the base includes a generally flatupper surface, the insert includes a generally flat lower surface, andthe generally flat upper surface contacts the generally flat lowersurface during the engaging step.
 15. A method of coupling a glenoidassembly to a glenoid surface of a scapula, the glenoid surface having asuperior portion and an inferior portion, comprising the steps of:providing a metal base of the glenoid assembly, the metal base includinga lip extending about a portion of an upper surface of the metal baseand defining a channel; coupling the metal base to the glenoid surface;providing an insert of the glenoid assembly, the insert being configuredto contact a head portion of a humeral component, and further the inserthaving a tab extending about a portion of a lower surface of the insert;and engaging the tab within the channel so as to couple the insert tothe base, wherein (i) movement from the inferior portion to the superiorportion of the glenoid surface defines a superior direction, and (ii)the insert is moved in the superior direction during the engaging stepfrom the inferior portion toward the superior portion of the glenoidsurface.
 16. The method of claim 15, further comprising the step ofproviding loading forces to the glenoid assembly in the superiordirection with a muscle after the insert is coupled to the base.
 17. Themethod of claim 16, wherein the muscle is a deltoid muscle or a rotatorcuff muscle.
 18. The method of claim 15, wherein: the base includes agenerally concave upper surface, the insert includes a generally convexlower surface, and the generally concave upper surface contacts thegenerally convex lower surface during the engaging step.
 19. The methodof claim 15, wherein: the base includes a generally flat upper surface,the insert includes a generally flat lower surface, and the generallyflat upper surface contacts the generally flat lower surface during theengaging step.